Thyrite protective device



June 1, 1943. G. USSELMAN THYRITE PROTECTIVE DEVICE Original File d Oct.2, 1956 16 osc/lm 70R rig INVENTOR 0. L. USSELMAN 361! M ATTO N EYPatented June 1, 1943 UNITED STATES PATENT OFFICE THYRITE PROTECTIVEDEVICE George L. Usselman, Port Jefferson, N. Y., assignor to RadioCorporation of America, a corporation of Delaware 4 Claims.

This invention relates to an improved protective circuit and, moreparticularly, to a protective circuit for the protection of filamentbypass condensers and filament transformers generally employed in aradio transmitter.

This application is a division of application No. 103,643, filed October2, 1936, now Patent #2,201,211, granted May 21, 1940.

Filament bypass condensers and transformers have heretofore beenmanufactured with a sufficiently high safety factor above its normalworking voltage so that it was not generally necessary to employ anyprotective device to protect the condensers and transformers againstoccasional increased voltages. However, with the advent of mercury vaportube rectifiers now employed in the higher powered transmitters, troublewith condenser and transformer failure has greatly increased due tohigher voltage surges. Several attempts have been made to protect thecondensers and transformers, such as to shunt spark gaps in rare gasacross the condensers for protection, but this method has not proven tobe reliable.

In the present invention, the filament bypass condensers and filamenttransformers are protected by novel circuits employing a particular formof material, sold under the name Thyrite, which is described more fullyin United States Patent #1322342, to McEachron, of September 8, 1931,and the General Electric Review for February, 1930, pages 92 to 100.(vol. 33, No. 2).

The use of this Thyrite protective device permits the use of lowervoltage bypass condensers, and consequently larger capacity can beobtained in the same size condenser case.

The present invention will best be understood by referring to theaccompanying drawing which is a wiring diagram of a portion of a radiotransmitter circuit wherein the bypass condensers are protected byThyrite discs.

The drawing shows an abbreviated transmitting circuit employing aprotective device I, which is made up of alternate plates or discs 2 ofThyrite and metallic electrodes 3, 4 and 5, the electrodes 3 and 4 eachbeing attached to a terminal of filament bypass condensers 6 and l whichare to be protected. The other electrodes 5 and 5' are connected toground. From the circuit, it will be noted that the Thyrite is inparallel with the apparatus to be protected. In this case, the centerpoints of the condensers 6 and l are connected to ground. The twooutside terminals of the condensers are connected to the filament leadsof the tube 8. The purpose of the bypass condensers 6 and l is to act asa radio frequency bypass to prevent the radio frequency from enteringand damaging the filament trasformer 9. It is also necessary that thefilament leads be grounded for radio frequency currents for the properoperation of a transmitter.

During the operation of high power transmitters employing mercury vaportubes there frequently occurs an arc-over, due to various causes, acrossthe neutralizing condenser l0 or in the tube 8, itself. In either case,the result is an instantaneous dischargeof the smoothing condenser H ina high voltage rectifier 12. This discharge enters the filament of tube8 and when protective devices are not used the current of the dischargemust flow through the secondary winding of the filament transformer 9,through the impedance 13 to ground. The impedance 13 may be suchapparatus as overload relays, meters, resistors, etc.

In any case, the impedance I3 is usually high enough to cause abnormalvoltages to occur from the terminals of condensers 6 and 1 to ground. Itis not practical for certain reasons, such as space requirement and highmanufacturing cost, to make the voltage rating of the condensers highenough to sustain more than about one-tenth of the full rectifiervoltage. However, in practical operation, condensers 6 and l are subjectto several times their voltage rating when an arc-over, as mentionedabove, occurs. This results in frequent condenser and filamenttransformer failures which continue to occur unless proper means ofprotection is used. A modulated source of high frequency oscillations I4is connected to the grid coupling condenser l5 and ground l6.

'Ihyrite has the property of a decrease of resistance with an increaseof current through it, which follows the law given more fully in theFebruary, 1930, issue of General Electric Review, and the McEachronPatent #1,822,742, of current=KV where V is the voltage across theThyrite material, and K is some constant depending upon the size andtype of the Thyrite plate. When the voltage is doubled, the currentthrough the Thyrite plate increases about twelve times so that it willbe seen that this material is ideal for a condenser protective device,and with proper mounting will protect the bypass condensers and.filament transformers from dangerous voltages which might causeinsulation failures of the transmitter, and as a consequenceinterruption of service.

Details of the mechanical construction of the Thyrite protectivestructure are shown and described in more detail in the application ofwhich this case is a division. Reference may be made to that case for adetailed description.

While I have have particularly shown and described several modificationsof my invention, it is to be particularly understood that my inventionis not limited thereto but that modifications may be made Within thescope of my invention.

I claim:

1. In a radio device, a thermionic discharge tube having at least a pairof filament terminals,

an energizing circuit connected thereto, means for effectivelyconnecting each of said terminals to ground for radio frequencycomprising a pair of condensers, each having one plate connected to oneof said filament terminals and the other plate grounded, and means forprotecting said condensers from voltage surges comprising an alternateassembly of Thyrite discs and metallic electrodes in intimate contactwith each other, the central and end electrodes of said assembly beingconnected together and con nected to ground, and connections from eachof the intermediate electrodes to said filament terminals.

2. In a radio device, a thermionic discharge tube having at least a pairof filament terminals, an energizing circuit connected thereto, meansfor effectively connecting each of said terminals to ground for radiofrequency comprising a pair of condensers, each having one plateconnected to one of said filament terminals and the other plategrounded, and means for protecting said condensers from voltage surgescomprising an alternate assembly of Thyrite discs and metallicelectrodes, the central and end electrodes of said assembly beingconnected together and connected to ground, and connections from each ofthe intermediate electrodes to said filament terminals.

3. In a radio device, a thermionic discharge tube having at least a pairof filament terminals and a plate, a source of high potential having oneterminal connected to said plate and. the other connected to a point ofcommon reference potential, a condenser connecting each of said filamentterminals to said point of reference potential, said condenser beingcapable of withstanding normal potentials thereacross but incapable ofwithstanding substantially the full potential of said source and meansfor protecting each of said condensers including a resistor connectedthereacross, said resistor having a negative resistance-voltagecharacteristic.

4. In a radio'device, a thermionic discharge tube having at least a pairof filament terminals and a plate, a source of high potential having oneterminal connected to said plate and the other connected to a point ofcommon reference potential, a condenser connecting each of said filamentterminals to said point of reference potential, said condenser beingcapable of withstanding normal potentials thereacross but incapable ofwithstanding substantially the full potential of said source and meansfor protectmg each of said condensers, said means including an alternateassembly of Thyrite discs and metallic electrodes in intimate contactwith each other, the central and end electrodes of said assembly beingconnected together and to said point of reference potential andconnections from each of the intermediate electrodes to said filamentterminals.

GEORGE L. USSELMAN.

